The institute of electrical and electronics engineers (IEEE) 802.16 standard provides a technique and protocol for supporting broadband wireless access. The standardization had been conducted since 1999 until the IEEE 802.16-2001 was approved in 2001. The IEEE 802.16-2001 is based on a physical layer of a single carrier (SC) called ‘WirelessMAN-SC’. The IEEE 802.16a standard was approved in 2003. In the IEEE 802.16a standard, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ are further added to the physical layer in addition to the ‘WirelessMAN-SC’. After completion of the IEEE 802.16a standard, the revised IEEE 802.16-2004 standard was approved in 2004. To correct bugs and errors of the IEEE 802.16-2004 standard, the IEEE 802.16-2004/Cor1 (hereinafter, IEEE 802.16e) was completed in 2005 in a format of ‘corrigendum’.
Recently, in the IEEE 802.16 Task Group j (hereinafter, IEEE 802.16j) based on IEEE 802.16e, a relay station (RS) is introduced to provide coverage extension and throughput enhancement, and a standardization thereof is being carried out. That is, according to the IEEE 802.16j standard, a signal can be delivered to a mobile station (MS) located outside a coverage of a base station (BS) via the RS, and a high-quality path employing high-level adaptive modulation and coding (AMC) can be configured for an MS located inside the coverage of the BS, thereby increasing system capacity with the same radio resource.
A representative operation mode in which the RS relays a signal between the BS and the MS includes an amplify-and-forward (AF) mode and a decode-and-forward (DF) mode. In the AF mode, a signal received from the BS or the MS is amplified and is then delivered to the MS or the BS. In the DF mode, information is restored by performing demodulation, decoding, or the like on the signal received from the BS or the MS, and thereafter a signal is generated by performing coding, modulation, or the like and is then delivered to the MS or the BS.
The AF mode has an advantage in that a time delay is short in signal delivery, but has a disadvantage in that noise is propagated or amplified in the delivered signal. On the other hand, the DF mode has an advantage in that noise can be removed from the signal, and reliability of signal transmission can be enhanced, but has a disadvantage in that a time delay may occur in signal delivery since a demodulation and decoding process is performed.
A multiple input multiple output (MIMO) system is a communication system using a plurality of transmit (Tx)/receive (Rx) antennas. The MIMO system can linearly increase channel capacity without additional increase of a frequency bandwidth when the number of Tx/Rx antennas increases. The MIMO technique includes a spatial diversity technique in which a symbol which has passed various channel paths is used to enhance transmission reliability and a spatial multiplexing technique in which a plurality of Tx antennas are used so that respective antennas simultaneously transmit separate data streams to increase a transfer rate.
For channel capacity increase and transmission reliability enhancement, an RS employing multiple antennas can be taken into consideration. When the RS operates according to the DF mode, a signal is received from a source station through multiple Rx antennas, and transmits a restored signal to a destination station through multiple Tx antennas. In this case, the conventional MIMO technique can be properly applied to the Rx antennas and the Tx antennas.
Unlike this, when the RS operates according to the AF mode, a connection relation between an Rx antenna and a Tx antenna is not clearly defined. Accordingly, there is a problem in that the conventional MIMO technique cannot be directly applied.